The ability to define patterns on a nanometer scale has applications in chemistry, biology, medicine, electronics, optics, material science, and other fields. In top-down fabrication processing, patterns are usually first produced in a resist film by various fabrication methods, including electron-beam lithography (EBL), extreme ultraviolet lithography (EUV), nanoimprint lithography, and block copolymer lithography, among others. The patterns can then be transferred into or onto the substrate using dry etching or lift-off
In these lithography techniques, normally the first step is to spin coat the resist film directly on the surface of the target substrate to be patterned. For spin-coating to work properly, surfaces should be flat at least on a millimeter scale. However, in a wide variety of emerging fields, for example, quantum photonics or fiber-integrated photonics, it is difficult to produce uniformly flat target substrates (e.g., diamond membranes or fiber facets), thereby rendering it challenging to fabricate high-resolution patterns on these materials using spin-coating based fabrication methods.
As a result, it can be challenging to make quantum photonic devices, which are often fabricated on 200-nm-thick diamond membranes. Generally it is hard to produce uniformly flat diamond membranes over a 100-μm scale. Also, it is inconvenient to process small diamond membranes with wet processing, such as resist coating, development, and acid etching, due to the difficulty in sample handling. In addition, the realization of high-quality photonic devices on such small diamond membranes usually involves nanometer precision, which is not easy to achieve by direct electron-beam lithography.
Several methods have been developed in recent years attempting to overcome the drawbacks of spin-coating and fabricate devices on unconventional substrates. For instance, focused ion beam (FIB) techniques can be used for nanofabrication, but FIB techniques can be slow and may result in undesired surface amorphitization, material redeposition, and gallium implantation. Several transfer methods involve moving sacrificial layers with metallic nano-patterns onto unconventional substrates, but these transfer methods lack controllable placement accuracy. Nanoimprint lithography with an ultraviolet (UV) curable resist may be applied for the fabrication of micro- or nano-patterns on the facets of optical fibers; evaporated negative resists for electron beam lithography (EBL) can be used for patterning on an atomic force microscope (AFM) cantilever and optical fiber, but they have limited resolution, e.g., about 50 nm.